Air-Conditioning Systems

Chill Out

There’s more to replacing or upgrading your air conditioning than meets the eye.

In this less-than-perfect economy, many people are keeping their current boats and upgrading them in various ways. One smart investment to improve your comfort and your boat’s future resale value is adding or upgrading air conditioning. This month and next I’ll discuss what to consider when choosing a system.

For a 40- to 80-foot boat, you’re best served by a split system (also called split-gas system), so named because the components are divided into two separate units, each located in a different location. The typical split system starts with a condensing unit—basically a compressor, seawater condenser, and electrical components—in the engine room. The advantage of placing it here is that noisy equipment is sequestered from your boat’s living areas.

Copper tubing carries refrigerant from the condensing unit to the other half of the system, the air-handling unit, which includes an evaporator coil and a blower. Air handlers are usually installed in the living areas, in the base of a berth or a lounge. One condensing unit can serve two air handlers and cool multiple cabins or a single large area. But split systems are limited by the number of condensing units that can be accommodated spatially and by a maximum run length of 50 feet for the copper tubing.

For boats more than 80 feet long, select the chilled-water approach. These systems operate via a chiller in the engine room that cools fresh water and pumps it through insulated piping to air handlers in the living spaces that in turn expel cool air. Typically, there is no reasonable limit on the number of air handlers a given system can support and only the units that are cooling a given area need be in use at any one time.

After selecting the A/C system you need, it’s time to figure out how many Btus it takes to cool your boat. Separate her interior into three areas. The below-deck region includes cabins, heads, and often the saloon and/or galley. The mid-deck region, often called the main deck, has small or shaded windows. Finally, the above-deck region typically has large glass surfaces and lots of direct sunlight.

Determine the square footage of each region by measuring the length and width of each space to cool or heat. When measuring a space that is narrower at one end than at another (like a bow stateroom), take your measurements across the middle. Since headroom is about 6 feet on the average boat, you usually don’t need to take it into account. Simply multiply length and width measurements for each space for the square footage then add all the square footage up for the region’s total.

The table below will help you determine what load factors to then use. Multiply the square footage of each region by the appropriate load factor and add similar calculations for other regions, and you’ll have the required capacity in Btus. A temperate climate, by the way, is typically one with maximum air temperatures of about 95F, water temperature of 85F, and moderate humidity. A tropical climate boosts air and water temperatures by 10F and raises humidity to high.

After establishing your Btu requirements, determine the number of air conditioners needed and their locations based on the size and layout of your boat, as well as the space limitations that will affect ducting and plumbing. The maximum ducting run is typically 15 feet, but make sure to reduce the overall length to ensure good airflow if bends are involved. You also need to consider which areas will require independent thermostat control, extra ducting and/or a secondary air handler. Finally, you can increase or reduce airflow with an adjustable grille and/or fan-speed control.

In the next article we determine the location of the units aboard your boat and the actual components needed.